Method of preparing water-dispersible polyolefin fibers and product formed therefrom

ABSTRACT

1. A METHOD OF FORMING WATER-DISPERSIBLE POLYOLEFIN FIBERS COMPRISING VAPORIZING SOLVENT FROM AN AQUEOUS MASS OF SOLVENT-CONTAINING POLYOLEFIN FIBERS, IN THE PRESENCE OF BETWEEN ABOUT 0.1% AND ABOUT 10% BY WEIGHT POLYVINYL ALCOHOL BASED ON THE DRY WEIGHT OF THE FIBERS.

U.S. Cl. 260-897 B 13 Claims ABSTRACT OF THE DISCLOSURE A method of forming water-dispersible polyolefin fibers comprising vaporizing solvent from an aqueous mass of solvent-containing polyolefin fibers, in the presence of polyvinyl alcohol, and the pulp and papers resulting from such a process.

BACKGROUND OF THE INVENTION This application is a continuation-in-part of United States patent application Ser. No. 168,383, filed Aug. 2, 1971, now abandoned.

In U.S. patent applications Ser. No. 27,053, filed Apr. 9, 1970, now abandoned, and refiled as continuation-inpart Ser. No. 235,420, filed Mar. 16, 1972; and Ser. No. 69,194, filed Sept. 3, 1970, now abandoned, and refiled as continuation-in-part Ser. No. 235,416, filed Mar. 16, 1972, processes are disclosed for forming polyolefin fibers which are useful for making synthetic paper. In the processes disclosed in both of these applications, a fibrous polyolefin gel is formed by operating under specified process conditions, and this gel is made up of fibers swollen by reaction solvent. The fibrous gel is subjected to shear stress, such as by the use of a disc refiner, while it is yet in the presence of reaction solvent, to develop the fibers. The fibers, as they emerge from the refiner, are still swollen with reaction solvent.

Other processes of preparing polyolefin fibers or fibrous materials by solvent precipitation techniques may result in solvent-swollen fibers that require further processing to remove the solvent in preparation for papermaking. Other techniques may also be employed in the production of polyolefin fibers which are solvent-swollen; for example, a process wherein fibers and fibrous materials are produced by forming a dispersion or emulsion comprising a polyolefin dissolved in a good solvent therefor as a first phase and water which is a poor solvent for the polyolefin as the second phase, the dispersion or emulsion being formed at an elevated temperature and pressure, and discharging the dispersion or emulsion into a zone of reduced pressure to provide fibers and fibrous materials. Such processes are disclosed, for example, in U.S. patent applications Ser. No. 295,339, filed Oct. 5, 1972; and Ser. No. 285,386, filed Aug. 30, 1972, now abandoned.

It is desirable to use synthetic fibers, such as those obtained from the processes just specified, to make synthetic fiber paper by utilizing conventional papermaking techniques. To use such techniques, the solvent desirably is removed from the fibers. Solvent exchange methods can be used to exchange the reaction solvent to water, but this adds expense to the process. We have found that a more advantageous manner of removing the solvent is to drive it off by vaporizing it, such as by using steam distillation techniques. We have also found, however, that problems of fiber agglomeration and irreversible loss of the fibrous character of the fibers are encountered in subjecting the solvent-swollen fibers to conventional steam distillation,

"United States Patent 3,848,027 Patented Nov. 12, 1974 ice such as by subjecting the fibers while in the solvent to direct steam sparging or to indirect heat.

Another problem that is encountered in using polyolefin fibers to make synthetic paper by conventional papermaking techniques resides in the fact that these fibers have a specific gravity less than water and they have a hydrophobic surface and are not wetted by water. These fibers tend to immediately float when placed in a body of water, wherein conventional papermaking requires the fibers to be dispersible in water.

SUMMARY OF THE INVENTION It is, therefore, a primary object of the present invention to provide a process for preparing water-dispersible polyolefin fibers from solvent-swollen polyolefin fibers by solvent vaporization techniques in a manner such that the fibers retain their fibrous character and are dispersible in water, and to provide a product formed from such fibers.

The above and other objects can be achieved by immersing the solvent-swollen polyolefin fibers in an aqueous solution of polyvinyl alcohol and then heating the fibercontaining slurry sufficiently to vaporize and thereby remove the solvent leaving the solvent-free fibers dispersed in the aqueous solution. The above and other objects can also be achieved by adding polyvinyl alcohol to a dispersion or emulsion of polyolefin in a good solvent therefor which has been added to water resulting in a two-liquid phase dispersion or emulsion at an elevated temperature and elevated pressure, the dispersion or emulsion being discharged into a reduced pressure zone whereby the polyolefin is recovered as fibrous material dispersed in aqueous solution and the solvent is substantially vaporized. Preferably, the polyvinyl alcohol is present in the water phase when the dispersion or emulsion is formed. The fibers can then be dewatered to wet cake form which is readily redispersible in water and a water leaf can be formed from these fibers in using these fibers to form synthetic paper by conventional papermaking techniques.

DESCRIPTION OF PREFERRED EMBODIMENTS The present invention may be utilized to treat any mass of solvent-swollen polyolefin fibers, such as polyethylene or polypropylene, from which it is desired to remove the solvent by vaporization techniques such as steam ditsillation.

Exemplary of suitable reaction solvents in which the polyolefin fiber may have initially been formed are such hydrocarbon solvents as hexane, cyclohexane, decaline, tetraline, chlorinated solvents, isooctane, m, o and pxylene, mineral oils, Socal 1 (a mixture of normal and branched aliphatic hydrocarbons, naptha and aromatics), and mixtures thereof. Preferred solvents are such solvents as hexane and cyclohexane which may be generally characterized as polyolefin-swellable, water-immiscible and having a boiling point below C. at atmospheric pressure.

Fibers useful as starting materials in this invention are any feltable polyolefin fiber or fiber-like material of a size and morphology useful in papermaking and wherein the fibers are swollen with solvent. Preferred sources of polyolefin fibers which may be treated in accordance with the present invention are such fibers as would be obtained from the processes specified in the aforementioned U.S. patent applications Ser. Nos. 27,053 and 69,194. Generally speaking, these fibers, after they have been suitably prepared for papermaking, are of papermaking size, i.e., about 0.2 to 3 millimeters in length, or larger; are formed from a polyolefin having an extremely high viscosity average molecular weight, which in the case of linear polyethylene and polypropylene is one having a melt index of from about zero to about 20 and a viscosity average molecular weight between about 20,000 and 20,000,000, or above; and have a surface area prior to and after removal of solvent, as measured by gas adsorption techniques, of from 1 to about 100 square meters/ gram or greater.

In accordance with the techniques specified in the aforementioned U.S. applications Ser. Nos. 27,053 and 69,194, a fibrous polyolefin gel is formed and passed through a device such as a disc refiner to form and liberate the fibers and develop fiber quality. The fiibers are in the presence of reaction solvent in the refiner; and, typically, the solvent-swollen fibers emerge from the refiner at 1-2% consistency, i.e., about 98-99% solvent and 12% fibers.

The mass of solvent-swollen fibers is then subjected to a procedure such as centrifuging or filtering to remove primarily nonsorbed solvent and form what may be described as a mass or cake of solvent-swollen fibers. In order to minimize the amount of polyvinyl alcohol based on fiber weight that need be used in a subsequent solvent-vaporizing operation it is desirable, in forming the solvent-swollen cake, to remove as much of the reaction solvent as can conveniently be removed. Typically, enough of the solvent is removed so that the mass of solvent-swollen fibers contains 6-25% fibers and 9475% solvent.

In accordance with the present invention, the mass of solvent-swollen polyole-fin fibers is immersed in an aqueous solution containing polyvinyl alcohol to form a slurry with the polyolefin fibers dispersed therein. The polyvinyl alcohol could, of course, be added simultaneously with the fibers or be present on the fibers when they are added to water, it only being essential that the polyvinyl alcohol is present during the subsequent treatment, such as steam distillation, wherein the slurry is heated to vaporize the solvent and remove it from the fibers. If polyvinyl alcohol is not present during the solvent vaporization procedure, we have found that there is severe fiber agglomeration with subsequent irreversible loss of the fibrous characteristics of the fibers. Alternately, as mentioned previously, the fibers can be produced by forming a dispersion or emulsion of a polyolefin, wherein solvent for the polyolefin and water have an elevated temperature (130 C.160 C.) and pressure (autogenous, up to about kg./cm. and flashing the dispersion or emulsion into a zone of reduced pressure (generally atmospheric), whereby substantially all the solvent vaporizes, leaving an aqueous slurry of fibers. In this alternative, the polyvinyl alcohol is included in the dispersion or emulsion prior to flashing of the solvent.

In order to measure the effectiveness of the polyvinyl alcohol addition as it contributes to maintenance of good fiber properties during solvent removal by vaporization, an emulsibility index was devised. To obtain a numerical value for the emulsibility index a Wet cake of solventswollen fiber (85% solvent, 15% fiber) which contains 5 grams of fiber (on a dry basis) is placed in Water with a specified concentration of polyvinyl alcohol (weight percent of polyvinyl alcohol based on the dry weight of the fiber). The total volume of the slurry is adjusted by water addition to 500 ml. The slurry is agitated at slow speed in a Waring Blendor for 3 minutes. The slurry is then placed in a 500 ml. graduated cylinder, inverted four times, and placed on a level table top. The volume of clear liquid under the fiber slurry is recorded after 10, 30, 40, 50, 60, 80, and 120 seconds. The values are totaled, and the sum divided by 5. The answer is called the emulsibility index. The lower the number, the better the emulsibility, and the better the effectiveness of the slurry in contributing to the maintenance of good fiber properties during solvent removal by vaporization. Desirably, the emulsibility index is below about 400.

We have found that if as little as 0.1% by weight of polyvinyl alcohol based on the dry weight of the polyolefin fiber is present in the slurry during vaporization of the solvent, the resultant fibers, after the solvent has been removed, are satisfactory to form handsheets therefrom. Typically, there are 0.53% fibers based on the water volume during solvent vaporization. Thus, the polyvinyl alcohol concentration based on the water is typically 0.005% to 0.03% for the case Where 1.0% polyvinyl alcohol is added based on the dry fiber weight.

Surprisingly, concentrations of about 0.1% polyvinyl alcohol by Weight of dry fiber has been found to effectively maintain fiber quality during solvent removal and permits obtaining solvent-free fibers which are dispersible in water. Higher concentrations of up to about 10% by weight of dry fiber may be employed within the scope of this invention, as shown in Table 1 below. While even higher concentrations of polyvinyl alcohol may be employed, such higher concentrations do not produce significant improvement. Preferably, between about 0.1% and 5% polyvinyl alcohol by weight of dry fiber is employed.

Because polyvinyl alcohol tends to foam at higher consistencies, we have found it desirable, when the concentration of polyvinyl alcohol is above about 3.0%, based on the dry weight of the fibers, to add a small amount (about 0.2% based on dry fiber weight) of a defoaming agent to the aqueous polyvinyl alcohol solution to prevent excess foaming during solvent removal. The type of defoaming agent is not critical, and We have used one readily available from Hercules, Inc., and called Hercules Deformer 831. Other defoamers such as Colloid 513, available from Colloid, Inc., and Surfynol 104 (tetramethyl decynediol) available from Air Reduction Company, Inc., could, of course, be used.

Polyvinyl alcohol is generally prepared commercially by hydrolyzing polyvinyl acetate, and polyvinyl alcohol with different degrees of hydrolysis are available.

While all grades of polyvinyl alcohol which we have tried have given satisfactory performance, we have found that certain grades are particularly desirable, especially when lower ranges of concentration based on fiber weight are used. The polyvinyl alcohols having a degree of hydrolysis greater than and viscosity (centipoise, 4% aqueous solution at 20 C.) greater than 2 appear to be particularly efl ective in the ranges of about .25 up to about 3.0% based on the dry weight of the fiber being treated.

The solvent vaporization process is carried out with the solvent-swollen mass of fibers in water, and the water may be heated by direct steam sparging of the solvent-swollen mass or by indirect heat.

There is nothing particularly critical about the vessel which is used to conduct the solvent vaporization process, it only being necessary that there be means to heat the slurry containing the solvent-swollen fibers in the aqueous polyvinyl alcohol solution and, desirably, there is means to provide at least gentle agitation to the slurry. If desired, the slurry can be made up in a vessel separate from the heating vessel and then the slurry transferred either batch-wise or by continuous flow through one or a number of heating vessels.

To optimize ultimate fiber quality, the temperature during vaporization should be maintained below about C., and, preferably, below 80 C. Therefore, if the boiling point of the solvent is greater than 85 C., the vaporization should be conducted under pressure conditions below atmospheric so that vaporization of the solvent can be accomplished without exceeding the stated temperature of 85 C. It may be necessary to add additional water to the slurry during vaporization of the solvent, especially if the boiling point of the solvent is higher than the boiling point of the aqueous solution.

After the reaction solvent has been removed from the polyolefin fibers, the aqueous solution, with the fibers dispersed therein, is filtered, pressed or centrifuged to remove much of the water and obtain a Wet fiber cake containing about 50% fibers. Surprisingly, by this invention the polyvinyl alcohol becomes intimately bound to the fibers and this can be seen because after the fibers have been dried they are redispersible in water. In fact, we have found that good dispersibility of the fibers in water was mamtained even when ten grams of fiber were continuously rinsed with a total of 60 gallons of water at a rate of three gallons aminute,

As mentioned above, dispersibility of the fibers in water is essential if conventional papermaking techniques are going to be utilized to form synthetic paper from these fibers.

;To measure the extent to which the fiber is dispersible in cylinder, inverted 'four times and placed on a fiat table top-The volume of clear water under the fiber slurry is recorded after 10, 20, 30, 40, 50, 60, 80, and 120 seconds.

. The values are summed, and the sum divided by 4 to give the dispersibility index. The lower the number, the better the dispersibility. To form an adequate sheet from the .fiberby conventional papermaking techniques, it is desirable that the dispersibility index number be below 350,

and, preferably, below 300.

Another factor that influences the ability of fiber to be satisfactorily formed into a sheet is the degree to which the fiber'has a tendency to fiocculate, which means its tendency to form fiber clumps. If there is a strong tendency to flo'cculate, or form fiber clumps, then, when the fibers are formed into paper, there will be a nonuniformity of fiber distribution,

In the language of papermakers, it is common to use the word formation to describe the fiber distribution properties in the formed web. A paper having good formation is when the nonuniformity of fiber distribution is the least. Paper made from fibers which have a strong tendency to' flocculate will be more apt to have poor formation.

Therefore, it was found to be desirable to devise a measurement called a flocculation index to measure the j tendency of fibers to flocculate. To obtain a numerical value for the flocculation index, 2 grams of fiber (dry weight) obtained from the wet cake formed after solvent removal were dispersed by agitating in a Waring Blendor for 5 seconds at high speed and were then added to enough water to make 800 ml. in an 800 ml. graduated cylinder.

The cylinder was inverted 4 times and placed on a level V table top for two hours. Two-hour time interval Was experimentally determined as being in excess of that time required to reach equilibrium, that is, far all of the fiber to float to the top. After two hours, there is a slurry phase on top and a clear water phase below it. The flocculation int d'ex is calculated as the volume in milliliters occupied by the slurry on top divided by the actual dry weight of the 'fiber in grams, The higher the number for the flocculation index, the greater the tendency of the fiber to flocculate.

The reason that the above is felt to be a valid estimanon ofthe flocculation tendency of the fibers is because if the fibers group into large flocs, then the total volume excluded by the slurry phase will be greater than if no flocs are formed. When handsheets were formed, as reported in the example below, the formation of the sheets by visual inspection to determine flocs showed a good correlation with the flocculation index.

EXAMPLE 1 A mass of polyethylene fibers at 1% consistency (99% solvent) is obtained from a disc refiner in accordance with the. teaching of the aforementioned US. patent application Ser. No. 69,194, the fibers having been formed in and swollen with hexane as a reaction solvent.

The fiber consistency was increased to 15% solvent) by filtering to form a wet mass of solvent-swollen fibers. Samples of this wet cake containing 5 grams (dry basis) fiber are prepared. An aqueous solution of polyvinyl alcohol is poured into a steam-jacketed Waring Blendor, and the solution contains a specified percentage and type of polyvinyl alcohol based on the dry Weight of the fiber, as reported in Table 1 below. The S-gram sample of fiber is added and the total volume of the slurry is adjusted to 500 ml. An emulsibility index reading is taken under the procedure set forth above, as is reported in Table 1 below. A condenser is connected to the mouth of the Blendor, and the slurry is gently agitated. The slurry is heated indirectly by admitting steam to the steam jacket surrounding the Blendor, and the heating continues until essentially all of the hexane has been driven from the slurry through the condenser. The temperature that is in the Blendor will remain at the boiling point of the solvent (which, in the case of hexane, is about 70 C.) until essentially all of the solvent is gone. An observation of the temperature rising above the boiling point of the solvent is an indication that the solvent has been removed and so the vaporization is halted. The aqueous fiber-containing slurry is filtered to obtain a 50% fiber cake and then dispersibility index and flocculation index readings are taken, according to the procedures specified above and reported in Table 1 below.

A water leaf in the form of handsheets are formed from the polyethylene fiber which have been prepared according to the procedures specified above. Sheetmaking and couching were performed as specified in TAPPI Standard T205 M-58, except that 800 ml. of stock at 0.25% consistency was used to make handsheets having a basis weight of about 60 pounds/3,000 square feet. The sheets are subjected to a 15-second cold (70 F.) press at 50 p.s.i. against a polished caul. The sheets are given a second identical press after turning them over on the caul so that both sides of the handsheet are given a smooth finish. The final drying is done on a rotary dryer at 220 F. The sheets are then tested in accordance with standard TAPPI testing methods.

In Table 1 below, the Elvanol 70-05 is a Du Pont polyvinyl alcohol having a viscosity of 4-6 and hydrolysis of 100-98.5%. Elvanol 71-30 is a Du Pont polyvinyl alcohol having a viscosity of 28-30 and hydrolysis of 100- 99%. Gelvatol 1/ is a Monsanto polyvinyl alcohol having a viscosity of 55-65 and a hydrolysis of 99-100%. Gelvatol 20/30 is a Monsanto polyvinyl alcohol having a viscosity of 4-6 and a hydrolysis of 87.7-89%. Gelvatol 20/60 is a Monsanto polyvinyl alcohol having a viscosity of 21-25 and a hydrolysis of 89-87%. Gelvatol 20/90 is a Monsanto polyvinyl alcohol having a viscosity of 34-45 and a hydrolysis of 89-87%. Gelvatol 40/10 is a Monsanto polyvinyl alcohol having a viscosity of 1.3- 2 and a hydrolysis of 77-72.9%. All of the viscosity values just reported are in centipoise (4% aqueous solution of 20 C.), and the hydrolysis is in mole percent.

Further explaining Table 1 below, the concentration of the polyvinyl alcohol is in percent based on the dry weight of the solvent-swollen fibers in the aqueous slurry during solvent removal. The EI, DI and F1 are, respectively, the emulsibility index, the dispersibility index and the flocculation index obtained by the procedures specified above. The tensile, stretch and TEA (tensile energy absorption) were obtained from the handsheets and measured in accordance with TAPPI Test T494 08-70. At all concentrations of 3.5% and above, about .2% based on dry fiber weight of a defoaming agent (Hercules Defoamer 831) was added to the slurry prior to solvent vaporization.

TABLE 1 Concentration, Tensile Stretch TEA, Polyvinyl alcohol percent EI DI F1 'inch percent ft..-lb./lt.

None 800 800 C) C) Elvanol Gelvatol:

specified above.

Because the E1. of the Gelvatol1/90, 20/30, 20/60, 20/90 and 40110 was high at low cpncsntration, greater agitation had to be used during solvent removal, but by using such agitation a fiber was obtained tially less than fiber obtained without any polyvinyl alcohol.

TABLE 2 Concentration, 151'. 2d 36. 4th Polyvinyl alcohol percent D1 D1 DI DI Gelvatol:

While the foregoing specification has set forth embodimerits of the invention in detail, for purpose of making a complete disclosure, various other embodiments and modifications will occur to those skilled in the art, but will fall within the spirit and scope of the invention defined in the following claims.

We claim:

from which a handsheet could be iormed because it had a DI substan 1. A method of forming water-dispersible polyolcfin fibers comprising vaporizing solvent from an aqueous mass of solvent-containing polyolefin fibers, in the presence of between about 0.1% and about 10% by weight polyvinyl alcohol based on the dry weight of the fibers.

2. The method as set forth in claim 1 wherein the polyvinyl alcohol has a viscosity (cp 4% at 20 C.) greater than 2, and a degree of hydrolysis greater than (mole percent).

3. The method as set forth in claim 1 wherein the solvent-swollen fiber mass added to the solution contains at least 6% fibers on a dry weight basis.

4. The method as set forth in claim 1 wherein the solvent is polyolefin-swellable, water-immiscible, and has a boiling point lower than C. at atmospheric pressure.

5. The method as set forth in claim 1 wherein the polyolefin is selected from the group consisting of polyethylene and polypropylene.

6. The method as set forth in claim 5 wherein the fibers are polyethylene which has a melt index of about 0 to about 20.

7. The method as set forth in claim 4 wherein the solvent is hexane.

8. The method as set forth in claim 1 wherein the polyvinyl alcohol has a degree of hydrolysis greater than about 72% (mole percent).

9. The method as set forth in claim 1 wherein the mass is immersed in an aqueous solution of polyvinyl alcohol to form a slurry of said fibers.

10. A method of forming water-dispersible polyolefin fibers comprising contacting a mass of solvent-swollen polyolefin fibers with water, in the presence of between about 0.1% and about 10% by weight polyvinyl alcohol based on the dry weight of the fibers, and heating the mass to vaporize and remove the solvent from the fibers to obtain substantially solvent-free fibers having improved water dispersibility and suitable for making a water leaf therefrom.

11. A method according to claim 10 wherein the presence of polyvinyl alcohol is between about 0.1% and about 5% by weight based on the dry weight of the fibers.

12. A method according to claim 10 wherein the polyolefin is selected from the group consisting of polyethylene and polypropylene.

UNITED STATES PATENTS 3,245,934 4/1966 Krzyszkowski 260-296 PM FOREIGN PATENTS 2,000,728 9/ 1969 France.

1,206,257 9/ 1970 Great Britain.

JOSEPH L. SCHOFER, Primary Examiner A. HOLLER, Assistant Examiner US. Cl. X.R.

162157 R; 26029.6 PM, 29.6 WA, 93.7, 94.9 GD, 94.9 E 

1. A METHOD OF FORMING WATER-DISPERSIBLE POLYOLEFIN FIBERS COMPRISING VAPORIZING SOLVENT FROM AN AQUEOUS MASS OF SOLVENT-CONTAINING POLYOLEFIN FIBERS, IN THE PRESENCE OF BETWEEN ABOUT 0.1% AND ABOUT 10% BY WEIGHT POLYVINYL ALCOHOL BASED ON THE DRY WEIGHT OF THE FIBERS. 